When it is sought to increase the density of information recorded on an optical disk, this objective is generally limited by the performance of the information read device. The basic principle is that the physical information written to the disk can only be read with great difficulty when its size is smaller than the resolution limit of the optical system which will be used to read this information. Typically, when reading with a red laser having a wavelength of 650 nm and a numerical aperture of 0.6, there is normally no hope of correctly reading information having a size below 0.4 microns, or at the limit 0.3 microns.
However, methods known as super-resolution methods have been devised for reading information whose physical size is smaller than, or even much smaller than the wavelength. These methods are based on the non-linear optical properties of certain materials. The expression “non-linear properties” is understood to mean the fact that certain optical properties of the material change depending on the intensity of the light that they receive. The read laser itself will locally modify the optical properties of the material via thermal, optical, thermooptical and/or optoelectronic effects on dimensions smaller than the dimension of the read laser spot; due to the change of properties, a piece of information present in this very small volume becomes detectable whereas it would not have been detectable without this change.
The phenomenon that is exploited is mainly based on two properties of the read laser that will be used:                on the one hand, the laser is very highly focused so as to have an extremely small cross section (of the order of the wavelength) but whose power distribution is Gaussian, very strong at its center, very attenuated at the periphery; and        on the other hand, a read laser power is chosen such that the power density over a small part of the cross section, at the center of the beam, significantly modifies an optical property of the layer, whereas the power density outside of this small cross section portion does not significantly modify this optical property; the optical property is modified in a direction that tends to allow information to be read which would not be readable without this modification.        
For example, the optical property which changes is an increase of the optical transmission in the case where the reading of one bit constituted by a physical mark formed on the optical disk requires a transmission of the laser beam to this physical mark. The non-linear layer is then interposed in the path of the beam towards the physical mark. The center of the laser beam will be able to pass through the layer as far as the mark due to the fact that by passing through the layer the intensity of the incident light makes said layer more transparent, whereas the periphery of the beam will not pass through as it does not modify the optical indices of the layer sufficiently to make it more transparent. Everything then takes place as if a beam focused over a much smaller diameter than that which its wavelength allows had been used.
Various theoretical propositions have been formulated to implement these principles, but none have given rise to an industrial development. Patent U.S. Pat. No. 5,153,873 recalls the theory. Patent U.S. Pat. No. 5,381,391 gives the example of a film having non-linear reflectivity properties. Patent U.S. Pat. No. 5,569,517 proposes various crystalline phase change materials.
Among the techniques which currently offer the strongest possibilities, there is the use of a platinum oxide (PtOx) layer held between two layers of a compound of zinc sulfide and silicon oxide, the assembly being inserted between two layers of an AgInSbTe or GeSbTe compound and the assembly being again inserted between layers of a compound of zinc sulfide and silicon oxide. The AgInSbTe or GeSbTe material has phase change properties under the effect of an intense laser illumination. Examples will be found in Applied Physics Letters, Vol. 83, No. 9, Sept. 2003, Jooho Kim et al., “Super-Resolution by Elliptical Bubble Formation with PtOx and AgInSbTe Layers”, and also in the Japanese Journal of Applied Physics, Vol. 43, No. 7B, 2004, Jooho Kim et al., “Signal Characteristics of Super-Resolution Near-Field Structure Disk in Blue Laser System”, and in the same journal, Duseop Yoon et al., “Super-Resolution Read-Only Memory Disk Using Super-Resolution Near-Field Structure Technology”. See also, Vol. 45, No. 2B, 2006, Hyunki Kim et al. “Random Signal Characteristics of Super-Resolution Near Field Structure Read-Only Memory Disk”.
The structures described in these articles mainly rely on the formation of platinum oxide expansion bubbles, trapped between the layers that surround them. These bubbles are formed during laser writing and they may be recognized during read-out, even with a read laser having a wavelength equal to several times the size of the bubbles.
But these structures are difficult to produce and the control of the bubble volume is particularly sensitive. The adjustment of the laser power in order to obtain the super-resolution effect when reading is also particularly difficult, too low a laser power does not give any result and too high a laser power considerably reduces the number of read cycles possible.